Diffusion af radioaktive isotoper fra bitumen ud i et omgivende ionadsorberende materiale

The differential equations describing the diffusion of radioactive isotopes from one phase into another phase with different characteristics are presented as well as the difference equations used for their numerical solution. The equations have been solved for the diffusion of some prominent radioactive isotopes
(Sr-90 and Cs-137) from bitumen into surrounding clay material. The problem arises as part of a safety evaluation of a proposed method for disposal of low- and medium level waste, where bitumen encapsulated waste is packed in a cement-clay material. Calculations have shown that the magnitudes of the effective diffusion coefficients in the packing material determine the extent of migration of radioactive isotopes into the surroundings, whereas the magnitudes of the diffusion coefficients in the encapsulating material have almost no influence on the migration range of the radioactive isotopes.